Pressure: Vacuums Are All About Nothing

by Bill Nye & Ian Saunders

Air pressure acts in all directions. But what

if there's no air at all?

When there's a place or a space with no air,

we call it a vacuum. Vacuum comes from an

old word that means "empty." Usually, things

that we call empty aren't really empty. They're

filled with air. An empty room may not have

any books or chairs or goldfish in it, but it's got

wall-to-wall air! And you know that air is made

up of molecules and that these molecules go

around bumping into things, making pressure.

A vacuum, though, is really empty. I mean

empty of everything—including air molecules.

And when there are no air molecules, we get

no air pressure.

A good example of a vacuum is a suction

cup stuck to a wall. When you push down on

the suction cup to attach it to the wall, you

force almost all of the air out from under it.

That creates a vacuum between the suction cup

and the wall. At the same time, the air pressure

in the room pushes against the other side of the

suction cup. The vacuum under the suction cup

has no air molecules and no air pressure, so

it doesn't push back. The air molecules in the

room hold the cup tightly to the wall.

The vacuum under a suction cup doesn't

last very long. Air molecules in the room are

constantly pushing against all sides of the

suction cup. Since there's a vacuum under the

suction cup, there's nothing to push back and

keep air molecules from getting through tiny

gaps between the wall and the cup. That means

that some of the air molecules from the room

can slowly sneak through into the space under

the suction cup. Eventually enough molecules

will sneak in, and the suction cup will fall off.

That's why we sometimes wet a suction cup

to make it stick better. The liquid fills in tiny

gaps between the cup and the wall. They're

small to us, but they're plenty big enough for

air molecules to slide through. See, it's harder

for air to get through the gaps when they're

filled with water, so the vacuum lasts longer.

Soda straws are another way to see a

vacuum in action. When you suck up a

milkshake, you suck most of the air molecules

from the straw into your mouth, making a

vacuum in the straw. Then the air pressure

in the room pushes down on the top of the

milkshake and pushes it up through your

straw. Right into your mouth. Mmmm!

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Of course, when people talk about a

vacuum, they usually mean a machine that

cleans the ﬂoor. But that kind of vacuum works

because it has a vacuum inside of it.

See, the motor inside the vacuum cleaner

blows almost all of the air out of the machine,

making a partial vacuum inside the machine.

Then, air in the room rushes in to fill the empty

space the only way it can—through the end of

the long hose you move over the carpet. All

that air rushing in carries dirt up off the ﬂoor.

Va-va-va-voom!

A.2.4.1

10. According to the passage, tiny gaps

between a suction cup and a wall will

A

B

C

D

keep the cup stuck tightly to the wall.

allow air molecules to get under

the cup.

force air molecules to push down

on the cup.

help create a vacuum between the cup

and the wall.

A.2.4.1

9.

Which statement best summarizes the

main idea of this passage?

A

A.2.3.1

11. In the passage, soda straws are used as

an example of how vacuums are

A

B

C

D

difficult to make.

created by liquids.

useful in everyday life.

ruined by air molecules.

B

C

D

"All that air rushing in carries dirt

up off the ﬂoor."

"The vacuum under a suction cup

doesn't last very long."

"And when there are no air

molecules, we get no air pressure."

"Eventually enough molecules will

sneak in, and the suction cup will

fall off."

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A.2.4.1

12. According to the passage, what is a

vacuum?

A

B

C

D

a space that has no air

a liquid pressing on air

a gap that allows air to pass

a collection of air molecules

A.2.4.1

14. Which would be the best new title for

this passage?

A

B

C

D

"How Vacuums Work"

"Why Vacuums Are Important"

"How to Use a Vacuum Cleaner"

"The History of the Vacuum

Cleaner"

B.3.3.1

13. Information in this passage is presented

through

A

B

C

D

expert opinions.

specific examples.

problems and solutions.

comparing and contrasting.